Contract Number: A933-078

Although sulfuric acid tends to dominate the atmospheric acidity in the eastern United States, nitric acid is more prevalent in southern California. In relation to sulfuric acid, little is known regarding the pulmonary toxicity of nitric acid, especially in environmentally relevant mixtures (such as with ozone). This study had two objectives: to examine the effects of inhaled nitric acid and ozone on lung macrophage cell function, and to evaluate new endpoints with respect to their applicability to future acid inhalation studies. The study was conducted in three parts: biological endpoint protocol development; a series of single, acute four-hour inhalation exposures; and a series of four days repeated four-hour inhalation exposures at 1/4 the acute exposure concentrations. The total exposure (concentrations of nitric acid and ozone x time) that the animals received was the same in the four-hour and the four-day experiments. Certified-viral-free Fischer 344 rats were exposed, nose-only; the rats were then sacrificed, and lung lavage was performed. Lung lavage cells and lavage fluid were evaluated using a variety of endpoints related to alveolar macrophage function. Alveolar macrophage-related endpoints were the focus of this study because alveolar macrophages are critically important cells with respect to defense against infection, sequestration of foreign material, hypersensitivity reactions, lung inflammation, and tissue destruction, all of which are known to be relevant to human health. The endpoints that were performed fell into six categories: 1) lavage cell number and type; 2) intracellular pH of lung lavage cells and lavage fluid pH; 3) lavage cell metabolism of the carcinogen precursor, benzo [al pyrene; 4) respiratory burst and phagocytic activity; 5) in vitro production of arachidonic acid metabolites (leukotriene B and leukotriene C) by lung lavage cells; and 6) elastolytic activity and elastase inhibitory capacity of lavage fluid and lung lavage cells. This study found effects of inhaled nitric acid on some of the above lung macrophage parameters, although the levels of acid used were greatly elevated over the ambient concentrations. A single four-hour exposure to 1 mg/m3 nitric acid resulted in increased production of leukotriene B, by lung lavage cells in vitro, and an increase in the elastase inhibitory capacity of lung lavage fluid. Exposure to 0.25 mg/m3 nitric acid four hours per day for four days resulted in decreased respiratory burst activity of lung lavage cells and, as in the single four-hour exposure, an increase in the lavage fluid elastase inhibitory capacity. Exposure to 0.6 ppm ozone for four hours had significant effects on a number of parameters such as lavage cell type, lavage fluid protein content, and elastase inhibitory capacity. However, for almost all endpoints examined, the interaction of nitric acid and ozone was not additive, in fact, a weak antagonistic interaction was observed.